U.S. patent number 6,074,776 [Application Number 09/006,015] was granted by the patent office on 2000-06-13 for polymerizable additives for making non-aqueous rechargeable lithium batteries safe after overcharge.
This patent grant is currently assigned to E-One Moli Energy (Canada) Limited. Invention is credited to Huanyu Mao, David Stanley Wainwright.
United States Patent |
6,074,776 |
Mao , et al. |
June 13, 2000 |
Polymerizable additives for making non-aqueous rechargeable lithium
batteries safe after overcharge
Abstract
After undergoing overcharge abuse, non-aqueous rechargeable
lithium batteries can be left in a relatively hazardous state of
charge, representing a safety concern with respect to subsequent
thermal or mechanical abuse. Electrolyte additives which
electrochemically form conductive polymers can be used to create a
short circuit inside the battery as a result of overcharge abuse
and automatically discharge the battery internally. The invention
is particularly suitable for batteries equipped with electrical
disconnect devices which cannot be discharged externally after the
disconnect has activated. Aromatic compounds such as biphenyl are
particularly suitable additives.
Inventors: |
Mao; Huanyu (Maple Ridge,
CA), Wainwright; David Stanley (Vancouver,
CA) |
Assignee: |
E-One Moli Energy (Canada)
Limited (Maple Ridge, CA)
|
Family
ID: |
4160687 |
Appl.
No.: |
09/006,015 |
Filed: |
January 12, 1998 |
Foreign Application Priority Data
|
|
|
|
|
May 16, 1997 [CA] |
|
|
2205683 |
|
Current U.S.
Class: |
429/61; 429/324;
429/57; 429/62; 429/328; 429/327 |
Current CPC
Class: |
H01M
10/0567 (20130101); H01M 10/4235 (20130101); Y02E
60/10 (20130101); H01M 2200/10 (20130101); H01M
6/50 (20130101) |
Current International
Class: |
H01M
10/42 (20060101); H01M 10/36 (20060101); H01M
10/40 (20060101); H01M 6/00 (20060101); H01M
6/50 (20060101); H01M 002/00 (); H01M 010/08 () |
Field of
Search: |
;429/324,327,328,57,61,62,185 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nuzzolillo; Maria
Assistant Examiner: Dove; Tracy
Attorney, Agent or Firm: Synnestvedt & Lechner LLP
Claims
What is claimed is:
1. A non-aqueous rechargeable lithium battery having a lithium
insertion compound cathode; a lithium compound anode; a non-aqueous
electrolyte; and a maximum operating charging voltage; wherein the
improvement comprises a monomer additive mixed in said electrolyte,
said monomer additive polymerizing to form an electrically
conductive polymer at battery voltages greater than the maximum
operating charging voltage, in combination with a battery
configuration designed so that said polymer creates an internal
short circuit in the battery during overcharge abuse independent of
any other overcharge protection means.
2. A non-aqueous rechargeable lithium battery as claimed in claim 1
wherein the battery additionally comprises overcharge protection
means to protect the battery against overcharge abuse wherein the
maximum operating charging voltage of the battery is exceeded.
3. A non-aqueous rechargeable lithium battery as claimed in claim 2
wherein the overcharge protection means comprises a positive
temperature coefficient resistor which increases in resistance
during overcharge abuse thereby limiting charging current.
4. A non-aqueous rechargeable lithium battery as claimed in claim 2
wherein the overcharge protection means comprises an internal
electrical disconnect device, said disconnect device activating at
a predetermined internal pressure; and a gassing agent other than
the monomer additive, said gassing agent generating gas and
pressure activating the disconnect device during overcharge
abuse.
5. A non-aqueous rechargeable lithium battery as claimed in claim 4
wherein the gassing agent is Li.sub.2 CO.sub.3.
6. A non-aqueous rechargeable lithium battery as claimed in claim 2
wherein the overcharge protection means comprises electrical
circuit means to limit charging current or voltage.
7. A non-aqueous rechargeable lithium battery as claimed in claim 1
wherein the mixture of electrolyte and monomer additive comprises
less than about 5% monomer additive by weight.
8. A non-aqueous rechargeable lithium battery as claimed in claim 1
wherein the monomer additive is aromatic.
9. A non-aqueous rechargeable lithium battery as claimed in claim 8
wherein the aromatic additive is biphenyl.
10. A non-aqueous rechargeable lithium battery as claimed in claim
9 wherein the mixture of electrolyte and aromatic additive
comprises about 2 to 3% biphenyl additive by weight.
11. A non-aqueous rechargeable lithium battery as claimed in claim
8 wherein the aromatic additive is selected from the group
consisting of pyrrole, N-methylpyrrole, thiophene, furan, indole
and 3-chlorothiophene.
12. A non-aqueous rechargeable lithium battery as claimed in claim
8 wherein the aromatic additive is pyrrole, N-methylpyrrole or
thiophene.
13. A non-aqueous rechargeable lithium battery as claimed in claim
8 wherein the aromatic additive is furan, indole, or
3-chlorothiophene.
14. A non-aqueous rechargeable lithium battery as claimed in claim
1 wherein the maximum operating charging voltage is greater than 4
volts.
15. A non-aqueous rechargeable lithium battery as claimed in claim
1 wherein the lithium insertion compound cathode is Li.sub.x
CoO.sub.2, Li.sub.x NiO.sub.2, or Li.sub.x Mn.sub.2 O.sub.4.
16. A non-aqueous rechargeable lithium battery as claimed in claim
1 wherein the lithium compound anode is a carbonaceous
compound.
17. A non-aqueous rechargeable lithium battery as claimed in claim
1 wherein the electrolyte solvent comprises an organic carbonate
selected from the group consisting of ethylene carbonate, propylene
carbonate, diethyl carbonate, and ethyl methyl carbonate.
18. A non-aqueous rechargeable lithium battery as claimed in claim
1 wherein the electrolyte solute comprises LiPF.sub.6 or
LiBF.sub.4.
19. A method for rendering an overcharged non-aqueous rechargeable
lithium battery safe to further abuse, the battery having a lithium
insertion compound cathode; a lithium compound anode; a non-aqueous
electrolyte; and a maximum operating charging voltage; which
comprises:
(a) selecting a monomer additive that polymerizes to form an
electrically conductive polymer at battery voltages greater than
the maximum operating charging voltage; and
(b) mixing an amount of the monomer additive in said electrolyte
wherein the amount is sufficient in combination with the battery
configuration such that an internal short circuit is created by the
polymerized additive during overcharge abuse thereby discharging
the battery to a safe state of charge independent of any other
overcharge protection means.
20. A method as claimed in claim 19 wherein the mixture of
electrolyte and monomer additive comprises less than about 5%
monomer additive by weight.
21. A method as claimed in claim 19 wherein the mixture of
electrolyte and monomer additive comprises sufficient monomer
additive to create an internal short circuit capable of discharging
the battery to a safe state of charge within 24 hours.
22. A method as claimed in claim 19 wherein the monomer additive is
aromatic.
23. A method as claimed in claim 22 wherein the aromatic additive
is biphenyl.
24. A method as claimed in claim 23 wherein the mixture of
electrolyte and aromatic additive comprises about 2 to 3% biphenyl
additive by weight.
25. A method as claimed in claim 22 wherein the aromatic additive
is selected from the group consisting of pyrrole, N-methylpyrrole,
thiophene, furan, indole and 3-chlorothiophene.
26. A method as claimed in claim 22 wherein the aromatic additive
is pyrrole, N-methylpyrrole or thiophene.
27. A method as claimed in claim 22 wherein the aromatic additive
is furan, indole, or 3-chlorothiophene.
28. A method as claimed in claim 19 wherein the maximum operating
charging voltage is greater than 4 volts.
Description
FIELD OF THE INVENTION
This invention pertains to non-aqueous rechargeable lithium
batteries and to methods for improving the safety thereof. It
particularly pertains to the use of polymerizable monomer additives
as means for rendering lithium ion batteries safe to further abuse
after the batteries have been overcharged.
BACKGROUND OF THE INVENTION
The demand for rechargeable batteries having ever greater energy
density has resulted in substantial research and development
activity in rechargeable lithium batteries. The use of lithium is
associated with high energy density, high battery voltage, long
shelf life, but also with safety problems (ie. fires), since
lithium is a highly reactive element. As a result of these safety
problems, many rechargeable lithium battery electrochemistries
and/or sizes are unsuitable for use by the public. In general,
batteries with electrochemistries employing pure lithium metal or
lithium alloy anodes are only available to the public in very small
sizes (eg. coin cell size) or are primary types (eg.
non-rechargeable). However, larger rechargeable batteries having
such electrochemistries can serve for military or certain remote
power applications where safety concerns are of somewhat lesser
importance, or the personnel involved are trained to deal with the
higher level of hazard.
Recently, a type of rechargeable lithium battery known as
lithium-ion or `rocking chair` has become available commercially
and represents a preferred rechargeable power source for many
consumer electronics applications. These batteries have the
greatest energy density (Wh/L) of presently available conventional
rechargeable battery systems (ie. NiCd, NiMH, or lead acid
batteries). Additionally, the operating voltage of lithium ion
batteries is often sufficiently high that a single cell can suffice
for many electronics applications.
Lithium ion batteries use two different insertion compounds for the
active cathode and anode materials. 3.6 V (average) lithium ion
batteries based on LiCoO.sub.2 /pre-graphitic carbon
electrochemistry are now commercially available. Many other lithium
transition metal oxide compounds are suitable for use as the
cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4.
Also, a wide range of carbonaceous compounds is suitable for use as
the anode material, including coke and pure graphite. The
aforementioned products employ non-aqueous electrolytes comprising
LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene
carbonate, propylene carbonate, diethyl carbonate, ethyl methyl
carbonate, and the like. Again, numerous options for the choice of
salts and/or solvents in such batteries are known to exist in the
art.
Lithium ion batteries can be sensitive to certain types of abuse,
particularly overcharge abuse wherein the normal operating voltage
is exceeded during recharge. During overcharge, excessive lithium
is extracted from the cathode with a corresponding excessive
insertion or even plating of lithium at the anode. This can make
both electrodes less stable thermally. The anode becomes less
stable as it gets doped or plated with reactive lithium while the
cathode becomes more prone to decomposing and evolving oxygen (see
J. R. Dahn et al., Solid State Ionics, 69(3-4), p265-270, 1994).
Overcharging also results in heating of the battery since much of
the input energy is dissipated as heat rather than stored. The
decrease in thermal stability combined with battery heating can
lead to dangerous thermal runaway and fire on overcharge.
Battery chargers and/or battery packs comprising assemblies of
individual lithium ion batteries are generally equipped with
appropriate electrical circuitry to prevent overcharge. However, in
the event of failure of the circuitry, many manufacturers
incorporate additional safety devices, in the individual batteries
themselves, to provide a greater level of protection against
overcharge abuse. For instance, as described in U.S. Pat. No.
4,943,497 and Canadian Patent Application Ser. No. 2,099,657, filed
Jun. 25, 1993, published Feb. 11, 1994, respectively, the lithium
battery products of Sony Corporation and Moli Energy (1990) Limited
incorporate internal disconnect devices which activate when the
internal pressure of the battery exceeds a predetermined value
during overcharge abuse. Various gassing agents (eg. cathode
compounds and/or other battery additives) may be used to generate
sufficient gas above a given voltage during overcharge so as to
activate the disconnect device.
Another alternative method relies on the net increase in internal
solids volume to hydraulically activate a disconnect device at a
specified state of overcharge (as disclosed in Canadian Patent
Application Ser. No. 2,093,763, filed Apr. 8, 1993, published Oct.
9, 1994).
Other overcharge safety devices may be incorporated in the lithium
batteries themselves to limit the charging current and/or voltage.
Positive temperature coefficient resistors (PTCs) are incorporated
by some manufacturers in part to limit the charging current during
overcharge abuse. These devices rely on a combination of heating of
the battery and IR heating of the PTC to trigger the PTC, which
thereby increases its resistance and limits the charging current.
In principle, it is also possible to consider incorporating an
electrical circuit for overcharge protection in the headers of the
individual batteries themselves.
These additional or backup safety devices can be effective insofar
as eliminating hazards associated with the electrical abuse of
overcharge. However, the overcharged battery is typically left in a
higher state of charge than normal. The contents of the battery can
therefore be left in a less than normal thermally stable state,
thereby posing more of a hazard than normal. Such overcharged
batteries can be more sensitive to subsequent mechanical abuse (eg.
being crushed) or thermal abuse (eg. being heated in an oven).
While many batteries can simply be discharged manually in the event
that overcharge abuse has occurred, thereby placing the battery in
a safe discharged state for later disposal, it is preferred that
this discharge be done automatically.
Batteries with activated internal electrical disconnect devices
however cannot be externally discharged to drain them of energy and
lower the state of charge. Such disconnected batteries may be
locked into an abnormally unsafe state of charge and pose
additional risk with regards to disposal or tampering.
Unfortunately, after the activation of a disconnect, such a battery
will appear to have no remaining capacity (ie. be completely dead).
At this point, an unwary consumer might be more tempted than usual
to disassemble or otherwise mechanically abuse the battery with
unfortunate consequences as a result. Thus, means for discharging
such overcharged batteries automatically and internally are highly
desirable.
Several means for automatically discharging batteries are known or
have been proposed in the art. Aqueous battery electrochemistries
may exhibit recombination reactions at the end of charge which
effectively serve to continuously discharge the battery while
charging continues. Additives (chemical shuttles) have also been
disclosed for non-aqueous battery electrochemistries to serve a
similar purpose. Recombination reactions and chemical shuttles may
be viewed as automatically discharging the batteries but only such
that the normal maximum operating charging voltage is not
exceeded.
Means for creating internal short circuits in overcharged batteries
are also known in the art. Electrochemical corrosion reactions may
be relied on to rapidly corrode metallic hardware or other
additives which are maintained at cathode potential (eg. cathode
current collector). A corroded species from the cathode can then
migrate and plate at the anode resulting in the formation of a
conductive dendrite. With continued corrosion and plating, a
conductive dendrite bridge can form between the cathode and anode
thereby electrically shorting the battery through the dendrite
bridge. Often, little actual charge needs to be consumed in
corrosion reactions before a dendrite bridge forms. Thus, cathode
hardware materials or other additives may be suitable for this
purpose if the onset of corrosion occurs above the maximum
operating voltage and if significant corrosion occurs before
overcharging presents a safety hazard. Many
readily available material options exist for low voltage (eg. circa
2 volt) non-aqueous batteries. For instance, in lithium
anode/molybdenum disulfide cathode batteries manufactured by Moli
Energy Ltd. in the 1980s, stainless steel and/or nickel hardware at
cathode potential would corrode, create dendrite bridges, and short
circuit the battery internally thereby limiting the state of charge
and protecting the batteries during overcharge abuse. However, not
so many material options are available for higher voltage (eg.
circa 4 volt) non-aqueous batteries. Most commonly available
hardware materials corrode at too low a potential to allow for the
normal operation of the battery. On the other hand, those
speciality materials which do not corrode at too low a potential
may not corrode significantly enough when needed for overcharge
protection. Thus, neither common nor speciality materials are
readily available for higher voltage non-aqueous batteries. `
Mechanical means for creating internal short circuits in
overcharged batteries have also been considered in the art. For
instance, one option proposed is similar to the aforementioned
electrical disconnect devices except that instead of effecting a
disconnect when activated, a mechanism would instead be
incorporated which effected a short circuit connection. This option
however is mechanically complex and raises cost and reliability
concerns.
Ideally, the means for creating internal short circuits on
overcharge would be reliable and inexpensive. Optimally, mild
shorts are produced, perhaps progressively or incrementally and
perhaps distributed throughout the inside of the battery, such that
the power and heat dissipated through the shorts is not suddenly
large or localized (ie. creating spot heating). Either of these
latter conditions represents a hazard in themselves.
Co-pending Canadian Patent Application Ser. No. 2,163,187, filed
Nov. 17, 1995, by a common inventor, discloses the use of
polymerizable monomer additives as gassing agents in lithium
batteries for purposes of activating internal electrical disconnect
devices on overcharge. Therein, it is disclosed that certain
monomer gassing agents which form conductive polymer products might
provide the additional advantage of creating an internal short and
discharging the batteries following overcharge abuse. In the
examples, this additional advantage is actually obtained in
batteries comprising a biphenyl additive. The polymerization
product of the biphenyl is conductive.
Co-pending Canadian Patent Application Ser. No. 2,156.800, filed
Aug. 23, 1995 by a common inventor, discloses the use of
polymerizable monomer additives for purposes of protecting a
rechargeable lithium battery during overcharge. Therein, a small
amount of polymerizable additive is mixed in the liquid
electrolyte. During overcharge abuse, the aromatic additive
polymerizes at voltages greater than the maximum operating voltage
of the battery thereby increasing its internal resistance
sufficiently for protection.
In the aforementioned co-pending Canadian patent applications Ser.
Nos. 2,163,187 and 2,156,800, it is not directly disclosed that it
would be advantageous in general to have batteries automatically
discharge themselves after overcharge abuse, ie. independent of
whether the battery contained an internal disconnect device. Also,
it is not directly disclosed that the use of monomer additives
which form conductive products when polymerized can be advantageous
independent of whether the monomer also serves as a gassing agent
or serves to significantly increase the internal resistance of the
battery.
Some aromatic compounds which are fundamentally capable of
polymerizing electrochemically and forming conductive polymers have
been used in electrolyte solvent mixtures and/or as electrolyte
solvent additives in certain specific rechargeable non-aqueous
lithium batteries for purposes of enhancing cycle life. In Japanese
Patent Application Laid-open No. 61-230276, a laboratory test cell
employing an electrolyte comprising a furan (an aromatic
heterocyclic) solvent additive demonstrated an improved cycling
efficiency for plated lithium metal. In Japanese Patent Application
Laid-open No. 61-147475, a polyacetylene anode, TiS.sub.2 cathode
battery employing an electrolyte comprising a thiophene solvent
additive showed better cycling characteristics than similar
batteries without the additive. No mention is made in these
applications about potential safety advantages resulting from the
electrochemical polymerization capability of the additives. Also,
it is unclear whether the actual embodiments in these applications
would possess a safety advantage in practice during overcharge
abuse as a result of incorporating the additives (ie. other events
that occur during overcharge might prevent polymerization and/or
polymerization might not result in the creation of an internal
short).
SUMMARY OF THE INVENTION
The invention comprises both methods and embodiments for
automatically discharging non-aqueous rechargeable lithium
batteries internally after the batteries have been subjected to
overcharge abuse. (Overcharge abuse is considered to occur when the
battery is charged to a voltage exceeding the normal maximum
operating charging voltage.) Monomer additives which form
electrically conductive polymer products when polymerized are
incorporated into the non-aqueous electrolyte. During overcharge
abuse, the monomer additive polymerizes thereby creating an
internal short circuit in the battery and discharging it.
The invention can be useful whether or not the batteries need to be
individually equipped with additional overcharge protection means.
For instance, low rate batteries may not require additional means
to ensure that the batteries are safe against electrical overcharge
abuse. However, after overcharge, such low rate batteries may still
pose a hazard with respect to subsequent thermal abuse. Thus, the
invention can be useful in cases where discharging these low rate
batteries to a lower state of charge renders them safer to
subsequent thermal abuse.
In a like manner, the invention can be useful for batteries
equipped with positive temperature coefficient (PTC) resistors or
other electrical circuit means to limit charging current or
voltage. Such batteries typically can be manually discharged at a
controlled rate to render them safer, if necessary. However, in
safety matters, it can be preferable to do this automatically and
internally to ensure that the discharging is indeed performed.
Certain additives of the invention, such as biphenyl, can not only
serve to automatically discharge an overcharged PTC equipped
battery, but can also serve to assist the PTC during the overcharge
by increasing the internal impedance (as disclosed in co-pending
Canadian Patent Application Ser. No. 2,156,800 above).
A preferred application of the invention is in rechargeable lithium
batteries which comprise an internal electrical disconnect device
wherein the disconnect device is activated at a predetermined
internal pressure. As in the aforementioned Canadian Patent
Application Ser. No. 2,163,187, the monomer additive may serve both
as the activating gassing agent and as the monomer which creates
the internal short circuit when polymerized. However, the monomer
additive of the instant invention need not be a primary source of
pressure activating gas, nor in fact a gassing agent at all. For
such embodiments, it may instead be desirable to employ other means
for activating the electrical disconnect device in combination with
the instant monomer additive. Since the internal short circuit can
be created by the instant additive after a partial overcharge (ie.
overcharge stops before activation of the electrical disconnect
device), the overcharged battery can be discharged and rendered
safe even if the partial overcharge abuse is not otherwise noticed
to have occurred by the activating of the disconnect.
Generally, the non-aqueous rechargeable batteries of the invention
comprise a lithium insertion compound cathode, a lithium compound
anode (eg. lithium metal, lithium alloy, or lithium insertion
compound), and a non-aqueous electrolyte (typically a liquid, but
polymer or plasticized polymer electrolytes may also be possible).
For lithium ion batteries, the lithium insertion compound cathode
can be Li.sub.x CoO.sub.2, or alternately can be selected from the
group consisting of Li.sub.x NiO.sub.2 and Li.sub.x Mn.sub.2
O.sub.4. The lithium compound anode can be a carbonaceous insertion
compound. The liquid electrolyte solvent can comprise organic
carbonates such as ethylene carbonate, propylene carbonate, diethyl
carbonate, and ethyl methyl carbonate. The electrolyte solute can
comprise various lithium salts such as LiPF.sub.6 or LiBF.sub.4.
The invention is particularly suitable for batteries whose maximum
operating charging voltage is greater than 4 volts.
Batteries of the invention additionally have a monomer additive
mixed in the electrolyte wherein the monomer additive polymerizes
at battery voltages greater than the maximum operating voltage
thereby forming a conductive polymer and creating an internal short
circuit in the battery. The amount of monomer additive must be
sufficient such that the polymer formed does indeed bridge both
cathode and anode thus shorting the battery. Amounts of less than
about 5% monomer additive by weight in the mixture of electrolyte
and monomer additive can be sufficient.
The monomer additive can be aromatic. Biphenyl is a particularly
suitable additive for lithium ion batteries with operating voltages
in the 4 volt range. Biphenyl can be effective in amounts of about
2 to 3 % by weight in the electrolyte mixture.
Aromatic heterocyclic compounds can also be suitable as additives.
For instance, pyrrole, N-methylpyrrole, and thiophene polymerize
and create an internal short in certain batteries. Thus, broadly
speaking, these additives are potentially suitable, but appear
preferable for use in batteries with maximum operating charging
voltages less than about 4 volts. Additives such as furan, indole,
and 3-chlorothiophene may be potentially suitable additives for
batteries with higher operating charging voltages. The substitution
of different chemical groups in these compounds is expected to
result in slight modifications to the polymerization potential
and/or conductivity of the product polymer. Thus, substituted
versions of these compounds may also be suitable and/or
preferred.
A method for obtaining the desired results in a given battery
embodiment involves selecting a monomer additive that polymerizes
to form an electrically conductive polymer at battery voltages
greater than the maximum operating charging voltage, and mixing an
amount of this monomer in the electrolyte wherein the amount is
sufficient such that an internal short circuit is created by the
polymerized additive during overcharge abuse thereby automatically
discharging the battery to a safe state of charge. Since neither
very rapid nor very slow discharge rates are desirable and since
the additive may serve no other purpose, the lowest enabling amount
of additive may be preferred as long as the internal short circuit
created is capable of discharging the battery to a safe state of
charge within about 24 hours. (Of course, additives like biphenyl
may serve other useful purposes in combination, such as activating
disconnect devices or increasing battery impedance as mentioned in
the two co-pending Canadian patent applications Ser. Nos. 2,156,800
and 2,163,189 above.)
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a cross-sectional view of a preferred embodiment of
a cylindrical spiral-wound lithium ion battery.
FIG. 2 shows the capacity versus cycle number data for the battery
in Example III.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
To minimize accidents, any energy storage device at the end of its
useful life is ideally drained of stored energy prior to disposal
and certainly prior to any action constituting abuse of the
battery. Non-aqueous rechargeable batteries are no exception,
particularly those used by the general public. Most non-aqueous
electrochemical systems used by the public require protection
against excessive charging since such electrical abuse usually
results in unwanted reaction products and by-product heat at a time
when the battery is fully loaded with energy. While these batteries
are adequately protected against overcharge itself, the batteries
can constitute a potential hazard if abused again thereafter (known
as `stacked abuse`).
It is preferable not to have to rely on the voluntary actions of
the public to ensure safety. Battery packs may be disassembled by
the public thereby removing external protection devices, and/or
individual batteries may be subsequently abused notwithstanding
warning notices and well publicized risk in so doing. Inoperative
batteries are perhaps more likely to be so casually treated by the
public than are batteries with some remaining life. In this regard,
the need to rely on the consumer can be desirably bypassed if
inoperative batteries would automatically discharge themselves
internally at a safe rate after overcharge.
Lithium ion batteries generally become less stable to thermal abuse
as the state of charge is increased. The upper voltage limit for
certain commercial lithium ion batteries is specified in part for
purposes of limiting the battery to a relatively safe state of
charge thermally. Such lithium ion batteries in an overcharged
state are fundamentally more prone to venting and catching fire
than other types of batteries. Thus, lithium ion chargers and/or
battery packs are typically equipped with reliable external
circuitry to prevent overcharge. However, this external circuitry
can be defeated by a determined user and even the most reliable
circuitry has a small but finite failure rate. Consequently,
internal overcharge protection devices are typically provided in
commercial lithium ion batteries. These devices are useful in
providing protection even if external circuitry is defeated or has
failed.
Preferably, once an internal overcharge protection device has been
activated, a lithium ion battery is no longer used. Until it is
discharged, the overcharged battery can pose a potential hazard
with regards to subsequent thermal or mechanical abuse.
Unfortunately, in order to effect a subsequent discharge, it is
often necessary to have the user intervene appropriately. In some
circumstances, it may not even be possible for the user to
discharge the battery externally. This is the case when internal
electrical disconnect devices have been activated. A battery with
an activated disconnect device appears to the user to be "dead" and
cannot be discharged externally.
The instant invention addresses this concern by providing automatic
means for discharging a non-aqueous battery internally after the
battery has been overcharged. This is accomplished by incorporating
a small amount of a suitable polymerizable monomer additive in the
non-aqueous electrolyte of the battery. The monomer additive is
selected such that it polymerizes at a suitable voltage to form an
electrically conductive polymer product. Significant polymerization
should not occur during normal operation of the battery (ie. in the
normal operating voltage range). However, during overcharge abuse,
the polymerizing voltage is attained whereupon the monomer additive
starts to polymerize. Eventually, enough conductive polymer is
formed to create a conductive bridge between the battery electrodes
thereby creating an internal short circuit in the battery and
discharging it. Preferably, the conductive bridge forms before the
battery ever reaches an undesirable state of charge from a safety
perspective. In that way, the battery cannot even be partially
overcharged to that undesirable state without initiating an
internal discharge.
The monomer additive must therefore meet several requirements
simultaneously in order to be effective. It must be capable of
polymerizing electrochemically to form an enabling conductive
polymeric bridge at a rather specific voltage. Also, the inclusion
of the additive must not otherwise adversely affect battery
performance. Although many monomers may work in principle, aromatic
monomers can be particularly suitable since the polymerization
potentials can be in a range that is suitable for this application
and the polymerization reactions can produce conductive products.
Additionally, aromatic compounds are often compatible with lithium
battery chemistries in small amounts.
As discussed in Organic Chemistry by R. J. Fessenden et al.,
Willard Grant Press, 1979, the term aromatic refers to a class of
ring compounds that are substantially stabilized by pi-electron
delocalization. Such compounds
are cyclic, planar, and each atom in the ring has a p orbital
perpendicular to the plane of the ring (Sp.sup.2 -hybrid state).
Also, the ring system must have 4n+2 pi electrons where n is an
integer (the Huckel rule). The term heterocyclic (see The Condensed
Chemical Dictionary 9th Ed., G. G. Hawley, Van Nostrand Reinhold,
1977) denotes a closed-ring structure, usually of either 5 or 6
members, in which one or more of the atoms in the ring is an
element other than carbon (eg. sulfur, oxygen, and nitrogen.)
Aromatic compounds in general have ring structures that can be
fairly easily polymerized electrochemically in a voltage range
suitable for the instant application. The presence of foreign atoms
in the ring structure of many aromatic heterocyclic compounds makes
the neighboring carbon atoms electron rich and hence the ring
structure is easily opened and polymerized at these locations.
Other unsaturated ring compounds do not polymerize as easily
electrochemically.
Examples of aromatic compounds which form conductive polymers
include biphenyl, pyrrole, indole, thiophene, furan, and
derivatives thereof. Table 1 (reproduced from Electrochemistry in
Organic Synthesis, J. Volke & F. Liska, Springer-Verlag, 1994)
shows the oxidation potentials of some example monomers versus a
standard calomel electrode and the electric conductivity of the
polymeric films formed.
TABLE 1 ______________________________________ (reproduced from
Electrochemistry in Organic Synthesis, J. Volke & F. Liska)
Compound Oxidation potential (V vs SCE) Conductivity (S cm.sup.-1)
______________________________________ pyrrole +0.8 30-100 indole
+0.8 5 .times. 10.sup.-3 -10.sup.-2 thiophene +0.9 10-100 furan
+1.85 10-80 ______________________________________
It should be noted that polymerization potentials depend to some
extent on the electrodes and other electrolyte components employed
in the electrochemical system. Literature values thus are useful
for suggesting potential compound candidates for the instant
application, but polymerization may proceed somewhat differently in
the actual battery environment. Thus, a compound may be suitable if
it polymerizes at voltages above the maximum operating charging
voltage of the battery but below the overcharge voltage at which
the battery becomes relatively hazardous under actual battery
conditions. Note that polymerization must also proceed it a
sufficient rate to result in enough polymer to form an adequate
bridge by the time it is needed.
The conductivity requirements of the polymerized polymer depend to
some extent on the morphology of the polymerized product and on the
battery electrochemistry and design. A dense polymeric conductive
bridge can be expected to have a lower resistance than a highly
fibrous bridge. Batteries having thick separators and/or small
electrode areas might require a polymer with greater conductivity
than batteries having thin separators and/or large electrode areas
since the same net resistance can be obtained with a more resistive
polymer and a shorter length, larger cross-sectional area bridge.
Finally, the internal resistance needed depends on specific battery
voltage, capacity, and state of charge versus relative hazard
characteristics.
Generally, for purposes of the invention, the lowest amount of
monomer additive is employed to effect the desired internal short.
While the additives must be relatively inert in the first place
with respect to lithium and to the electrodes (ie. should not be
capable of reacting with the lithium or inserting in the
electrodes) excessive amounts of even an inert additive may be
expected to adversely affect battery performance characteristics
(eg. by increasing battery impedance). Typically, for operation of
the invention, amounts of the order of a few percent by weight or
volume in the electrolyte is sufficient. The actual amount required
for enablement will again depend in part on battery
electrochemistry and design as well as the monomer
characteristics.
Several criteria must therefore be met when choosing additives for
a given application. Although the acceptable ranges for meeting
these criteria may be relatively broad, some non-inventive
empirical trials are required in order to verify the suitability of
a particular additive candidate for any given battery application.
These trials would be expected to include overcharge testing of
trial batteries comprising varied amounts of additive candidate.
Either during or after the selecting of an apparently enabling
amount of an additive, some performance testing of trial batteries
is also required to completely test for adverse effects on
performance. Such trials should be well within the scope and
capabilities of those skilled in the art, and not require inventive
input.
We have found that biphenyl is a particularly preferred additive
for use in typical commercial lithium ion battery products for
consumer electronics. These batteries typically have thin
separators (about 25 micrometers thick) and high surface area
electrodes (circa a few hundred square centimeters). Battery
capacities of order of 1Ah and up are common. The normal maximum
operating charging voltages are about 4.2V. Between this limit and
about 5 volts, the batteries become relatively more hazardous.
During overcharge at C rate or more, a few percent of biphenyl
additive can polymerize sufficiently to form a conductive bridge to
discharge the battery to a safe state of charge within 24 hours. As
disclosed in Canadian Patent Application Ser. No. 2,156,800, the
biphenyl additive appears to polymerize at 4.70 volts vs
Li/Li.sup.+ in such battery environments and use of a small amount
does not adversely affect battery performance significantly. Other
potentially suitable additives such as 3-chlorothiophene and furan
were also identified therein.
In the Examples to follow, other additives have been identified
which might be suitable for use in non-aqueous batteries having
lower operating charging voltages (ie. less than 4.2V). These
additives include pyrrole, N-methylpyrrole, and thiophene and seem
more suitable for lower voltage batteries because internal shorts
are formed at too low a voltage in representative example
batteries.
It is expected that other additives which are closely related to
the preceding (ie. substituted compounds or derivatives thereof)
will show similar but slightly modified properties and thus may be
a preferred choice for certain applications.
With the exception of the presence of the additive, the
construction of batteries of the invention can be conventional.
Generally, an enabling amount of additive is simply mixed in with
the bulk electrolyte at some preferred point during normal
assembly. Minor handling changes may of course be required to
account for differences in the properties of the bulk electrolyte
and the additive (eg. vapor pressure, toxicity, etc.).
Non-aqueous rechargeable lithium batteries appear in various
configurations commercially (ie. prismatic formats or miniature
coin cells) and many different components may be used. (For
instance, while such additives would likely be less mobile in a
polymeric electrolyte, it is conceivable that batteries comprising
solid polymer electrolytes might achieve similar benefits by
incorporating such additives.) A preferred construction for a
lithium ion type product is depicted in the cross-sectional view of
a conventional spiral-wound battery in FIG. 1. A jelly roll 4 is
created by spirally winding a cathode foil 1, an anode foil 2, and
two microporous polyolefin sheets 3 that act as separators.
Cathode foils are prepared by applying a mixture of a suitable
powdered (about 10 micron size typically) cathode material, such as
a lithiated transition metal oxide, possibly other powdered cathode
material if desired, a binder, and a conductive dilutant onto a
thin aluminum foil. Typically, the application method first
involves dissolving the binder in a suitable liquid carrier. Then,
a slurry is prepared using this solution plus the other powdered
solid components. The slurry is then coated uniformly onto the
substrate foil. Afterwards, the carrier solvent is evaporated away.
Often, both sides of the aluminum foil substrate are coated in this
manner and subsequently the cathode foil is calendered.
Anode foils are prepared in a like manner except that a powdered
(also typically about 10 micron size) carbonaceous insertion
compound is used instead of the cathode material and thin copper
foil is usually used instead of aluminum. Anode foils are typically
slightly wider than the cathode foils in order to ensure that anode
foil is always opposite cathode foil.
The jelly roll 4 is inserted into a conventional battery can 10. A
header 11 and gasket 12 are used to seal the battery 15. The
external surface of the header 11 is used as the positive terminal,
while the external surface of the can 10 serves as the negative
terminal. Appropriate cathode tab 6 and anode tab 7 connections are
made to connect the internal electrodes to the external terminals.
Appropriate insulating pieces 8 and 9 may be inserted to prevent
the possibility of internal shorting. Prior to crimping the header
11 to the can 10 in order to seal the battery, electrolyte 5 is
added to fill the porous spaces in the jelly roll 4. In batteries
of the invention, the electrolyte 5 additionally comprises an
enabling amount of monomer additive.
The batteries are protected against the electrical abuse of
overcharge via one or more acceptable constructions such as:
pressure activated internal electrical disconnect devices, positive
thermal coefficient devices (PTC), or overcharge protection
circuitry. Additional safety devices can be incorporated for other
reasons if desired. Usually, a safety vent is incorporated that
ruptures if excessive pressure builds up in the battery.
The battery depicted in FIG. 1 is equipped with an internal
electrical disconnect device in the header 11 which is similar to
that shown in Canadian Patent Application Ser. No. 2,099,657. The
disconnect device can be activated by a gassing agent such as
Li.sub.2 CO.sub.3. The gassing agent may, but need not, also serve
as a polymerizable additive for creating an internal short (as
disclosed in Canadian Patent Application Ser. No. 2,163,187 above).
It may instead be preferred to employ a polymerizable additive for
creating an internal short which does not generate gas during
overcharge and to use alternate means for activating the disconnect
device (eg. such as disclosed in Canadian Patent Application Ser.
No. 2,093,763 above). Monomer additives that polymerize via the
breaking of double bonds may not generate any gaseous by-products
and thus could be suitable for such a situation.
The following discussion is provided for purposes of illustration,
but should not be construed as limiting in any way. Without being
bound by theory, polymerization of the additive is believed to
occur at the cathode resulting in the formation of polymer on the
cathode surfaces. Additive throughout the electrolyte should
continue to migrate towards the cathode and polymerize or, contact
therewith resulting in the growth of a deposit which can eventually
extend through the separator and contact the anode. Thus, a
conductive bridge can be formed. In typical lithium ion batteries,
the electrodes are both in close physical contact with a thin, low
volume, microporous separator. As such, even a relatively small
amount of monomer might be expected to enable a desired internal
short.
The following Examples are provided to illustrate certain aspects
of the invention but should not be construed as limiting in any
way. 18650 size (18 mm diameter, 650 mm height) cylindrical
batteries were fabricated as described in the preceding and shown
generally in FIG. 1. Cathodes 1 comprised a mixture of LiCoO.sub.2
powder, a carbonaceous conductive dilutant, and polyvinylidene
fluoride (PVDF) binder uniformly coated on both sides of a thin
aluminum foil about 5.4 cm in width by 49.5 cm in length. Coating
weight was about 47 mg/cm.sup.2. Anodes 2 were made using a mixture
of a spherical graphitic powder plus Super S (trademark of Ensagri)
carbon black and polyvinylidene fluoride (PVDF) binder (in amounts
of about 2% and 10% by weight respectively to that of the spherical
graphitic powder) uniformly coated on thin copper foil of similar
length to the cathode but 3 mm greater in width. Coating weight was
about 23 mg/cm.sup.2. Microporous polyolefin film was used to form
the separators 3. The electrolyte 5 was a solution of a lithium
salt dissolved in a solvent mixture of ethylene carbonate (EC),
propylene carbonate (PC), and diethyl carbonate (DEC) in a
EC/PC/DEC volume ratio of 30/20/50. Approximately 5 cc of
electrolyte was used in each battery.
Example I
Two 18650 batteries were assembled as described above using a 1.5 M
LiBF.sub.4 electrolyte solution except that the first comparative
battery contained no additive while the second inventive battery
comprised 2% by weight biphenyl additive in the electrolyte.
(Biphenyl is a solid at room temperature and thus; is conveniently
quantified by weight rather than by volume.) These batteries were
also equipped with a pressure relief vent and internal electrical
disconnect device as described in the aforementioned Canadian
Patent Application Ser. No. 2,099,657. The batteries were initially
conditioned at 21.degree. C. by charging, discharging, and then
charging again to the normal maximum operating voltage of 4.1
volts.
Both batteries were then subjected to overcharge abuse at a
background temperature of 21.degree. C. using a current supply with
10 volt compliance. The batteries were partially overcharged at 3
and 3.6 amps respectively for 12 minutes (a time sufficient to
significantly raise the battery state of charge without activating
the internal electrical disconnect). The voltage of the batteries
was then monitored for about 19 hours. The voltage of the first was
stable over this period at about 4.5 volts. The voltage of the
second dropped continously to about 4.05 volts by the end of this
period. Each battery was then subjected to nail penetration abuse
which results in a hard internal short. The first comparative
battery vented explosively with flame. The second inventive battery
did not vent or burn.
This example shows that the battery comprising biphenyl additive,
even though originally overcharged slightly more than the
comparative battery, discharged itself sufficiently to be markedly
safer on subsequent mechanical abuse.
Example II
a) Ten 18650 batteries were assembled and conditioned as in Example
I except that a 1 M LiPF.sub.6 electrolyte solution comprising 2.5%
by weight biphenyl additive was employed. These batteries were
overcharged at 21.degree. C. at 3.6 amps until the internal
electrical disconnect device was activated. (The biphenyl
additionally served as a gassing agent in this example to activate
the disconnect device as described in the aforementioned Canadian
Patent Application Ser. No. 2,163,187). The batteries were stored
for 24 hours and then were subjected to nail penetration abuse. No
battery vented or burned. The maximum skin temperature recorded on
the batteries during nail penetration abuse was 33.degree. C.
b) Three 18650 batteries were assembled and conditioned as in a)
above except that no additive and no internal electrical disconnect
device was employed. Instead, these batteries were equipped with
PTC devices in the header to limit the charging current thereby
protecting the battery during overcharge abuse. These batteries
were overcharged at 21.degree. C. at 3.6 amps until the PTC
activated (ie. the PTC heated up sufficiently to increase suddenly
and markedly in resistance). The batteries did not vent or burn.
The batteries were next stored in an open circuit condition for 24
hours and then were subjected to nail penetration abuse. One of the
three batteries vented violently with flame.
c) Six 18650 batteries were assembled and conditioned as above
except that a 1.5 M LiBF.sub.4 electrolyte solution was employed
without any additive. The batteries were constructed such that the
internal electrical disconnect devices were hydraulically activated
by the net increase in internal solids volume at a specified state
of overcharge as described in Canadian Patent Application Ser. No.
2,093,763 above. These batteries were overcharged at 21.degree. C.
at 3.6 amps until the internal electrical disconnect devices
activated. The batteries did not vent or burn. The
batteries were next stored for 24 hours and then were subjected to
nail penetration abuse. Five of the six batteries vented violently
with flame.
This example shows that within 24 hours after overcharging,
disconnect equipped batteries comprising the additive were markedly
safer to subsequent mechanical abuse than comparative batteries
equipped with either disconnects or PTCs, but comprising no
additive.
Example III
A 18650 size battery was assembled as described in Example I except
that the electrolyte comprised 5% by weight biphenyl additive. The
battery was then charged to 4.1 volts and stored at 60.degree. C.
for one week. Thereafter, the battery was cycled at 21.degree. C.
using a constant 1 amp current discharge to 2.5 volts and a current
limited, constant voltage charge to 4.1 volts. Every 20 cycles, a
series of discharge currents with decreasing magnitude was applied
in a stepwise fashion to determine if any capacity loss was
recovered at a lower discharge rate. FIG. 2 shows the capacity
versus cycle life data for this battery.
This example shows that excellent cycling results can still be
obtained even with the presence of up to 5% by weight biphenyl
additive.
Example IV
A series of 18650 batteries was made similar to those of Example I
in order to screen potential candidates from a performance
perspective. In this series, batteries comprising the following
additives (% by volume) were made and electrically conditioned:
0.5% pyrrole, 0.42% N-methylpyrrole, and 1% thiophene. Batteries
comprising pyrrole additive developed such a significant internal
short during conditioning that they could not be fully charged,
implying, that the internal short carried more than the 60mA
charging current. The onset of shorting began circa 3.5 V and the
battery voltage did not exceed about 3.7 V. Batteries comprising
the N-methylpyrrole additive were charged to 4.1 V and were
monitored at open circuit thereafter. The voltage dropped
significantly, to about 3.9 V in 24 hours. An internal short
appears to have developed above about 3.5 V. A battery comprising
the thiophene additive was charged to 4.2 V and was noted to drop
to 4.09 V after one hour at open circuit.
While these additives appear unsuitable for use in the high voltage
battery embodiments of the previous Examples (because internal
shorts develop in the normal operating voltage range), they
nonetheless may be suitable additives for non-aqueous batteries
with lower operating charging voltages.
As will be apparent to those skilled in the art in the light of the
foregoing disclosure, many alterations and modifications are
possible in the practice of this invention without departing from
the spirit or scope thereof. Accordingly, the scope of the
invention is to be construed in accordance with the substance
defined by the following claims.
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